Mononitrobenzene is made industrially by mixing benzene and nitric acid in the presence of sulfuric acid as a reaction catalyst. The majority of mononitrobenzene is produced in adiabatic units using either a series of continuous stirred tank reactors (CSTR) or a plug flow reactor. In the adiabatic process, the heat of reaction is used to reconcentrate the spent sulfuric acid catalyst. Both the nitric acid and benzene reactants and the sulfuric acid are introduced at the reactor entrance.
When benzene and nitric acid are reacted to make mononitrobenzene, some well known by-products are formed, among them, nitrophenols and dinitrobenzene. These by-products are made, though at different levels, regardless of the technology used. The amount of nitrophenol formed is typically an order of magnitude higher than that of dinitrobenzene, and much research effort has been invested through the years in reducing this unwanted by-product.
Reducing the formation of dinitrobenzene has received less research attention. It is known by industrial producers of mononitrobenzene that the formation of dinitrobenzene can be slightly reduced by the introduction of a molar excess of benzene feed over nitric acid feed, and reduced somewhat more by changing the concentration of the catalytic sulfuric acid. The latter effect is described in U.S. Pat. No. 4,021,498 Alexanderson et al. For example, Alexanderson et al. indicates that higher concentrations of dinitrobenzene are produced if the sulfuric acid concentration is in excess of 72 wt %. The data of the Alexanderson et al. patent also suggest that changing the average reaction temperature affects the formation of dinitrobenzene. However a reduction of average reactor temperature is accompanied by a reduction in the kinetics of mononitrobenzene formation.
Dinitrobenzene formation consumes mononitrobenzene and nitric acid and thereby reduces the yield of product mononitrobenzene. Further, most industrial mononitrobenzene is hydrogenated to make aniline, and dinitrobenzene in mononitrobenzene is suspected of poisoning the catalyst of industrial aniline reactors.
When low dinitrobenzene concentrations are required in the product mononitrobenzene, the current industrial method used is distillation of the product mononitrobenzene. This process step is energy intensive, and requires a purge of dinitrobenzene with mononitrobenzene, which further lowers the yield of benzene to mononitrobenzene. However, it is used because there are currently no other practical methods available to produce industrial mononitrobenzene in adiabatic nitration with low dinitrobenzene concentration.